In order to reduce a leakage current through a gate insulation layer of a transistor, a method of forming the gate insulation layer using a high-k material has been developed. However, when a gate electrode including polysilicon is formed directly on the gate insulation layer including the high-k material, the transistor may have an increased threshold voltage and decreased electron mobility due to the Fermi-level pinning effect. Thus, a transistor having a gate electrode including a metal has been developed. However, a method of manufacturing a transistor having good electrical performance is still needed.